This paper explains and updates the results [19] given in the talk by the author at the conference "RIMS Seminar, Representation spaces, twisted topological invariants and geometric structures of 3-manifolds" held in 2012. Some of the results in [19] have already been shown in [18]. This paper also modifies some statements in [18,19] with some progress.This research is motivated by [12,15] to investigate geometric properties of the crosssection of the character variety of a knot with the hyperplanes defined by meridionally trace-free (traceless) conditions, which section we call the trace-free slice of the character variety of a knot (or of a knot, simply). In [18], we have given a set of equations associated with a diagram of a knot whose common solutions coincide with the trace-free slice. This gives us a strong tool to describe exactly the relationship between the trace-free slice, degree 0 abelian knot contact homology and the character variety of the 2-fold branched cover of the 3-sphere branched along the knot. Using this, we give a notion called ghost characters of a knot and conditions on them to check(1) when Ng's conjecture concerned with the above relationship does not hold, (2) when the map Φ : S 0 (K) → X(Σ 2 K) constructed [22] is not surjective. Recently, Christopher R. Cornwell [4] translated the results in [19] on the above relationship into the language of "reflective augmentation" by using [18,22].